| dc.description.abstract | This study evaluates the simulation of tropical precipitation by the Community Climate
Model, Version 3, developed at the National Center for Atmospheric Research. For
an evaluation of the annual cycle of precipitation, monthly-mean precipitation rates from
an ensemble of CCM3 simulations are compared to those computed from observations of
the TRMM satellite over a 44-month period. On regional and sub-regional scales, the comparison
fares well over much of the Eastern Hemisphere south of 10◦S and over South
America. However, model - satellite differences are large in portions of Central America
and the Caribbean, the southern tropical Atlantic, the northern Indian Ocean, and the
western equatorial and southern tropical Pacific. Since precipitation in the Tropics is the
primary source of latent energy to the general circulation, such large model - satellite differences
imply large differences in the amount of latent energy released. Differences are
seasonally-dependent north of 10◦N, where model wet biases occur in realistic wet seasons
or model-generated artificial wet seasons. South of 10◦N, the model wet biases exist
throughout the year or have no recognizable pattern.
For an evaluation of the diurnal cycle of precipitation, hourly-averaged precipitation
rates from the same ensemble of simulations and for the same 44-month period are compared
to observations from the Tropical Rainfall Measuring Mission (TRMM) satellite.
Comparisons are made for 15◦ longitude ?? 10◦ latitude boxes and for larger geographical
areas within the Tropics. The temporally- and spatially-averaged hourly precipitation rates
from CCM3 and from TRMM are fit to the diurnal harmonic by the method of linear leastsquares
regression, and the phases and the amplitudes of the diurnal cycles are compared.
The model??s diurnal cycle is too strong over major land masses, particularly over South
America (by a factor of 3), and is too weak over many oceans, particularly the northwestern
Tropical Pacific (by a factor of 2). The model-satellite phase differences tend to be
more homogeneous. The peak in the daily precipitation in the model consistently precedes
the observations nearly everywhere. Phase differences are large over Australia, Papua New
Guinea, and Saharan Africa, where CCM3 leads TRMM by 4 hours, 5 to 6 hours, and 9 to
11 hours respectively. A model sensitivity experiment shows that increasing the convective
adjustment time scale in the model??s deep convective parameterization reduces its positive
amplitude bias over land regions but has no effect on the phase of the diurnal cycle. | en |
